When our side has already bid three suits, tournament players usually agree to play a bid of the 4th suit as artificial. For example:

1H  1S
2C  2D

Playing 4th suit forcing, 2D here does not necessarily have diamonds. The fourth suit simply shows a good hand where no other bid accurately describes your hand. Typically it is a hand that wants to play in game, but has no strong opinion about which game to choose.

This is a convention, and only applies if you have previously agreed to play the convention with your partner.

In a Teams game, how much you beat your opponent by is more important. One overtrick is not very valuable, and your main goal is to make your contract. If you can find a way to guarantee your contract, it is usually worth sacrificing an overtrick to do so.

The theory that when a defender plays an honour, they are less likely to hold the adjacent missing honour.

As it applies to Andrew Robson's article in our January 2019 issue, you are looking at a double-finesse against the king-queen of hearts. Once the first finesse loses to the queen, it becomes more likely that the king will be onside.

Another way to look at it: it is more likely that one of the honours is onside, than that both are offside. 

If both declarer and dummy run out of a side suit, it can be dangerous for the defenders to lead that suit. Since declarer does not have to follow suit in either hand, it is possible for declarer to ruff in one hand while discarding a loser from the other hand.

In this way, declarer can get rid of a loser that would have been impossible to dispose of if he had been forced to follow suit.

A defender may deliberately give a ruff and discard, if they know that declarer has no losers to discard. In this case, making declarer ruff may be the defenders' best chance to get another trick (by weakening declarer's trumps).

A combination of the best and 3rd-best cards in a suit. This is typically the ace and the queen.

If the queen has been played already, the ace and jack are a tenace, etc.

A tenace can be used for taking a finesse. Alternatively, if you can get your opponents to lead into your tenace, then you don't have to take the finesse:
                 32
   K4                       J5
                 AQ

A finesse here would lose, but if West leads the suit you will make two tricks without needing the finesse.

A split tenace is when the two cards are in different hands:
                 A2
   K4                       J5
                 Q3

The ace and queen form a split tenace. A split tenace can't be used for taking a finesse (if South leads the queen, West will cover an honour with an honour, promoting a trick for his partner's jack).

However, if you can force West to lead the suit, you'll still make your extra trick. If West leads a small card away from his king, you can let it run around to your queen.

Ruffing with a high trump, forcing declarer to overruff with a higher trump, in order to help set up a trick for your partner.

For example:
                 765
   Q2                       J3
              AKT984

Left to his own devices, declarer will cash the AK and draw all the defenders' trumps. If East can arrange to ruff a side-suit with the jack, and declarer overruffs with the king, then West will later make a trick with the queen.

Note that East has to ruff with the jack to execute the uppercut. If East ruffs with the three, declarer will overruff with the four, and still be able to draw trumps with the ace and king.

The losing trick count will be featured in the October 2017 issue of the Novice magazine.